Objective 3

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

Objective 3

Describe and diagram the microscopic structure of skeletal muscle fibers.


Connective Tissue Wrappings of Skeletal Muscle Tendon – connects

muscle to bone Epimysium – covers

the entire skeletal muscle

Figure 6.1


Connective Tissue Wrappings of Skeletal Muscle Perimysium – around

a fascicle (bundle) of fibers

Endomysium – around single muscle fiber

Muscle Fiber = Muscle Cell

Figure 6.1


Microscopic Anatomy of Skeletal Muscle Sarcolemma – specialized plasma membrane Sarcoplasmic reticulum – specialized smooth

endoplasmic reticulum

Figure 6.3a


Microscopic Anatomy of Skeletal Muscle Cells are multinucleate Nuclei are just beneath the sarcolemma

Figure 6.3a

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin CummingsFigure 6.3b

Microscopic Anatomy of Skeletal Muscle Myofibril – small fiber of a muscle cell

Bundles of myofilaments Myofibrils are aligned to give distinct

bands I band =

light band A band =

dark band


Microscopic Anatomy of Skeletal Muscle Sarcomere

Contractile unit of a muscle fiber

Figure 6.3b


Microscopic Anatomy of Skeletal Muscle Organization of the sarcomere

Thick filaments = myosin filaments Composed of the protein myosin Has ATPase enzymes

Figure 6.3c


Microscopic Anatomy of Skeletal Muscle Organization of the sarcomere

Thin filaments = actin filaments Composed of the protein actin

Figure 6.3c


Microscopic Anatomy of Skeletal Muscle Myosin filaments have heads (extensions, or

cross bridges) Myosin and

actin overlap somewhat

Figure 6.3d


Microscopic Anatomy of Skeletal Muscle At rest, there is a bare zone that lacks actin

filaments Sarcoplasmic

reticulum (SR) – for storage of calcium

Figure 6.3d


Objective 4

Explain the events of muscle cell contraction.


Properties of Skeletal Muscle Activity Irritability – ability to receive and respond to

a stimulus Contractility – ability to shorten when an

adequate stimulus is received


Nerve Stimulus to Muscles Skeletal muscles

must be stimulated by a nerve to contract

Motor unit One neuron Muscle cells

stimulated by that neuron

Figure 6.4a


Nerve Stimulus to Muscles Neuromuscular junctions – association site of

nerve and muscle

Figure 6.5b


Nerve Stimulus to Muscles Synaptic cleft – gap

between nerve and muscle Nerve and

muscle do not make contact

Area between nerve and muscle is filled with interstitial fluid

Figure 6.5b


Transmission of Nerve Impulse to Muscle Sodium rushing into the cell generates an

action potential Once started, muscle contraction cannot be

stopped


Transmission of Nerve Impulse to Muscle Neurotransmitter – chemical released by

nerve upon arrival of nerve impulse The neurotransmitter for skeletal muscle is

acetylcholine


Muscle Contraction 1. Neurotransmitter Acetycholine released by

a neuron (signal)2. Calcium ions are released from the

sarcoplasmic reticulum3. Calcium ions bind to regulatory proteins4. Actin’s binding sites are exposed by

changed regulatory proteins5. Myosin head attaches to actin filament6. Actin pulled toward the center of the

sarcomere (the contraction)


Muscle Contraction (continued) 7. Calcium is removed, myosin and actin not

bonded anymore 8. ATP (energy) is needed to release Actin

and myosin filaments 9. Actin and myosin return to resting position


The Sliding Filament Theory of Muscle Contraction Activation by nerve

causes myosin heads (crossbridges) to attach to binding sites on the thin filament

Myosin heads then bind to the next site of the thin filament

Figure 6.7


The Sliding Filament Theory of Muscle Contraction This continued action

causes a sliding of the myosin along the actin

The result is that the muscle is shortened (contracted)

Figure 6.7


The Sliding Filament Theory

Figure 6.8


Contraction of a Skeletal Muscle Within a skeletal muscle, not all fibers may

be stimulated during the same interval Different combinations of muscle fiber

contractions may give differing responses Graded responses – different degrees of

skeletal muscle shortening


Objective 5

List factors that can influence the force, velocity, and duration of muscle contraction.


Types of Graded Responses Twitch

Single, brief contraction Not a normal muscle function

Figure 6.9a–b


Types of Graded Responses Tetanus (summing of contractions)

One contraction is immediately followed by another

The muscle does not completely return to a resting state

The effects are added

Figure 6.9a–b


Types of Graded Responses Unfused (incomplete) tetanus

Some relaxation occurs between contractions

The results are summed

Figure 6.9c–d


Types of Graded Responses Fused (complete) tetanus

No evidence of relaxation before the following contractions

The result is a sustained muscle contraction

Figure 6.9c–d


Muscle Response to Strong Stimuli Muscle force depends upon the number of

fibers stimulated More fibers contracting results in greater

muscle tension Muscles can continue to contract unless they

run out of energy


Energy for Muscle Contraction Initially, muscles used stored ATP for energy

Bonds of ATP are broken to release energy Only 4-6 seconds worth of ATP is stored

by muscles After this initial time, other pathways must be

utilized to produce ATP


Energy for Muscle Contraction Direct phosphorylation

Muscle cells contain creatine phosphate (CP)

CP is a high-energy molecule

After ATP is depleted, ADP is left

CP transfers energy to ADP, to regenerate ATP

CP supplies are exhausted in about 20 seconds

Figure 6.10a


Energy for Muscle Contraction Aerobic Respiration

Series of metabolic pathways that occur in the mitochondria

Glucose is broken down to carbon dioxide and water, releasing energy

This is a slower reaction that requires continuous oxygen

Figure 6.10b


Energy for Muscle Contraction Anaerobic glycolysis

Reaction that breaks down glucose without oxygen

Glucose is broken down to pyruvic acid to produce some ATP

Pyruvic acid is converted to lactic acid

Figure 6.10c


Energy for Muscle Contraction Anaerobic glycolysis

(continued) This reaction is not as

efficient, but is fast Huge amounts of

glucose are needed Lactic acid produces

muscle fatigue

Figure 6.10c


Muscle Fatigue and Oxygen Debt When a muscle is fatigued, it is unable to

contract The common reason for muscle fatigue is

oxygen debt Oxygen must be “repaid” to tissue to

remove oxygen debt Oxygen is required to get rid of

accumulated lactic acid Increasing acidity (from lactic acid) and lack

of ATP causes the muscle to contract less

Objective 3

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